Drilling muds are used to carry rock cuttings out of the wellbore and to the surface. Both water-based and oil-based muds are used as drilling muds. Water-based muds are generally less toxic than those based on oil but the latter possess many operational advantages, particularly for the drilling of high angle, long reach and high pressure/high temperature wells.
However conventional oil based muds also suffer from a number of undesirable characteristics. In particular oil may be retained on the drill cuttings resulting in adverse environmental considerations.
It is very important therefore that any oil retained on cuttings should be removed before disposal, particularly in drilling operations offshore where cuttings may be discharged to the sea, rivers or lakes and cause oil pollution e.g. of the marine environment. Alternatively they have to be transported onshore for disposal which can be costly and time consuming. Future legislation will also require current discharge levels to be reduced.
If the use of oil based drilling muds is to continue, despite more restrictive discharge legislation, efficient methods of reducing or preventing oil pollution on cutting discharge will be required.
A solution to the problem of reducing oil discharge into the sea would be the use of treatment facilities on an offshore platform to remove the oil and allow the discharge of clean cuttings into the sea.
Several methods have been used to remove oil from drill cuttings. In one method the oil based mud is washed from the surface of the cuttings by use of a fluid medium, for example a detergent. The cuttings are then discharged and the oil recovered from the wash fluid. A major limitation in this method is that oil internally absorbed into the pores of the rock cannot be removed and between 30 and 50% of the original oil may be retained.
Another method involves cleaning rather than washing the cuttings. This may be carried out by means of a solvent extraction system using, for example, toluene or methylene chloride as the solvent. Oil may be extracted from the cuttings and then removed from the solvent and reused. However the potential hazards caused by the toxic nature of the solvent chemicals have raised doubts about the method.